Autothrottle problems suspected in Heathrow 777 crash

Investigators probing last Thursday's Heathrow Boeing 777 crash may be able to glean useful information from six previous engine failures on the type, one of which could prove highly significant in pinpointing the cause of the incident.

The Air Accidents Investigation Branch (AAIB) has apparently ruled out bird strike and fuel starvation as factors in the accident, and its update says it's examining why "neither engine responded to throttle lever inputs during the final approach".

Although it was initially believed both engines on BA flight 038 had failed, it's now been revealed that they "did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust".

The AAIB elaborates: "As previously reported, whilst the aircraft was stabilised on an ILS approach with the autopilot engaged, the autothrust system commanded an increase in thrust from both engines.

"The engines both initially responded but after about 3 seconds the thrust of the right engine reduced. Some eight seconds later the thrust reduced on the left engine to a similar level."

An earlier single engine failure recorded by the US National Transportation Safety Board involved a Malaysian-registered 777 which on 2 August 2005 "suffered a loss of thrust while climbing half an hour after take off from Perth".

The plane retured safely to the airport, and Australian investigators subsequently "identified computer failings which led to the pilots being given inaccurate speed readings". The US Federal Aviation Administration "ordered a computer upgrade, warning that faulty data could cause difficulties with the flight controls, autopilot, pilot displays, brakes and autothrottles".

That BA038 may have suffered a similar computer-based problem is, of course, highly speculative pending the final outcome of the AAIB investigation. The AAIB notes: "All possible scenarios that could explain the thrust reduction and continued lack of response of the engines to throttle lever inputs are being examined, in close cooperation with Boeing, Rolls Royce and British Airways."

Boeing, meanwhile, is remaining tightlipped on the matter. A spokesman said: "The 777 has been in service for 12 years and has flown around 3.6 million flight hours during which there have been no fatalities. It would be inappropriate to comment at this stage." ®

Another possible cause?

Thanks very much to Ian Powell for pointing us in the direction of this analysis of "loss of thrust control" (LOTC) incidents on General Electric Company Model GE90 turbofan engines fitted to Boeing 777s.

Specifically, it describes how ice can cause "a corruption of the FADEC (full authority digital engine control) signal and result in abnormal engine start characteristics on the ground or lack of engine response to commanded thrust levels in flight".

While noting that BA038 was fitted with Rolls Royce powerplants, the article says the Federal Aviation Authority's Airworthiness Directive on the matter appears to "confirm that there are paths where FADEC signals can be corrupted", which may pose a threat to the operation of all 777 engines, regardless of type.